GHSA-W62V-XXXG-MG59
Vulnerability from github – Published: 2026-07-21 18:33 – Updated: 2026-07-21 18:33Summary
cx() in hono/css composes class names from plain strings but marks the result as already-escaped without HTML-escaping the input. When the result is used as a JSX class attribute during server-side rendering, the value is written into the attribute unescaped, so untrusted input can break out of the class attribute and inject arbitrary markup, leading to Cross-Site Scripting (XSS).
Details
Because the composed value is treated as pre-escaped, the HTML attribute escaping normally applied to interpolated values is skipped, and characters such as " pass through unescaped — allowing a value to terminate the attribute and add further attributes or elements. This arises when an application passes untrusted, user-controlled input as a class name to cx(), for example when merging a base class with an externally provided className.
Impact
During server-side rendering, an attacker who controls a value passed to cx() can inject arbitrary HTML into the page, resulting in stored or reflected XSS in the victim's browser.
This may lead to:
- Execution of attacker-controlled script in the victim's browser session.
- Session hijacking, credential theft, or actions performed on behalf of the victim.
Applications are affected only if they render JSX server-side and pass untrusted input as a class name to cx().
{
"affected": [
{
"package": {
"ecosystem": "npm",
"name": "hono"
},
"ranges": [
{
"events": [
{
"introduced": "4.0.0"
},
{
"fixed": "4.12.27"
}
],
"type": "ECOSYSTEM"
}
]
}
],
"aliases": [
"CVE-2026-59895"
],
"database_specific": {
"cwe_ids": [
"CWE-116",
"CWE-79"
],
"github_reviewed": true,
"github_reviewed_at": "2026-07-21T18:33:50Z",
"nvd_published_at": "2026-07-08T17:17:27Z",
"severity": "MODERATE"
},
"details": "### Summary\n\n`cx()` in `hono/css` composes class names from plain strings but marks the result as already-escaped without HTML-escaping the input. When the result is used as a JSX `class` attribute during server-side rendering, the value is written into the attribute unescaped, so untrusted input can break out of the `class` attribute and inject arbitrary markup, leading to Cross-Site Scripting (XSS).\n\n### Details\n\nBecause the composed value is treated as pre-escaped, the HTML attribute escaping normally applied to interpolated values is skipped, and characters such as `\"` pass through unescaped \u2014 allowing a value to terminate the attribute and add further attributes or elements. This arises when an application passes untrusted, user-controlled input as a class name to `cx()`, for example when merging a base class with an externally provided `className`.\n\n### Impact\n\nDuring server-side rendering, an attacker who controls a value passed to `cx()` can inject arbitrary HTML into the page, resulting in stored or reflected XSS in the victim\u0027s browser.\n\nThis may lead to:\n\n- Execution of attacker-controlled script in the victim\u0027s browser session.\n- Session hijacking, credential theft, or actions performed on behalf of the victim.\n\nApplications are affected only if they render JSX server-side and pass untrusted input as a class name to `cx()`.",
"id": "GHSA-w62v-xxxg-mg59",
"modified": "2026-07-21T18:33:50Z",
"published": "2026-07-21T18:33:50Z",
"references": [
{
"type": "WEB",
"url": "https://github.com/honojs/hono/security/advisories/GHSA-w62v-xxxg-mg59"
},
{
"type": "ADVISORY",
"url": "https://nvd.nist.gov/vuln/detail/CVE-2026-59895"
},
{
"type": "WEB",
"url": "https://github.com/honojs/hono/commit/cd3f6f7194f0e5c9d4b26ae0cf232018d0f388fc"
},
{
"type": "PACKAGE",
"url": "https://github.com/honojs/hono"
},
{
"type": "WEB",
"url": "https://github.com/honojs/hono/releases/tag/v4.12.27"
}
],
"schema_version": "1.4.0",
"severity": [
{
"score": "CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:C/C:L/I:L/A:N",
"type": "CVSS_V3"
}
],
"summary": "Hono: Server-Side XSS via JSX Escaping Bypass in cx() Utility"
}
Sightings
| Author | Source | Type | Date | Other |
|---|
Nomenclature
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- Not patched: The vulnerability was not observed as successfully patched by the user who reported the sighting.
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